PROBLEM
Many municipal water suppliers have changed from free chlorine to chloramine disinfection in order to reduce the formation of trihalomethanes (TTHMs) and Haloacetic acids (HAA5s), as well as maintain a sufficient disinfectant residual throughout the distribution system. However, chloramine systems can promote the growth of nitrifying biofilms, and tanks with poor water turnover are especially susceptible to nitrification. Regular free chlorine burnouts are effective at killing most of the nitrifying bacteria but do not physically remove the slime and scale from the tank surfaces. Thus, nitrifying biofilms regrow rapidly. Burnout chlorination can also lead to temporary accumulation of high TTHM and HAA5 levels.
SOLUTION
In 2007 Blue Earth Labs Top Ultra tank cleaning chemistry was used by a Missouri water system that was experiencing nitrification problems in a particular tank. The reservoir is a 500,000-gallon, glass lined tank with a history of significant nitrification. The tank was taken off-line for nine days to determine baseline levels for nitrite, nitrate, and total chlorine. The tank was emptied, cleaned with Top Ultra, filled with water and taken off-line to determine post cleaning nitrification levels.
Prior to Top Ultra cleaning, nitrite levels in the tank increased significantly after only five days offline (from 0.016 mg/L on day 2 to 0.109 mg/L on day 10). On day 10 the off-line period was terminated since the nitrite levels were approaching the maximum allowable levels. After the cleaning process, the nitrite levels remained relatively low throughout an 11-day period, increasing to 0.34 mg/L on day 11.
Only minor changes in nitrate levels were observed during both off-line periods. However, the average nitrate concentration in the tank was 1.8 mg/L before cleaning and 1.18 mg/L after cleaning. This reduction was consistent throughout the off-line periods.
Nitrification is often accompanied by a decline in total chlorine residual, due to biofilm growth. Prior to cleaning, the residual dropped by 0.7 mg/L over 9 days; after the cleaning, the reduction was only 0.2 mg/L over 11 days.
RESULTS
Overall, chemical tank cleaning with Top Ultra was very effective at removing the visible deposits from the interior tank surfaces, and the procedure resulted in significant reduction of nitrification and chlorine demand.